1. Technical Field
The present invention generally relates to spark plugs having a copper core side wire, and more particularly, to a spark plug wherein the side wire includes a metal sheath welded to a metal spark plug shell using a novel "roll-over" technique.
2. Discussion
Conventional spark plug side wires, also referred to in the art as ground electrodes or outer electrodes, include solid nickel based embodiments. As illustrated in FIG. 6, conventional solid nickel-based side wires 200 are normally butt welded with a high welding force ranging from 220 to 300 lbs. to the shell 210 of the spark plug 220. As can be seen from the weld profile for such a process, the solid nickel side wire 200 does not penetrate the steel shell 210 of the spark plug 220.
More recently, spark plug side wires have been provided with copper cores in order to enhance thermal conductivity. However, welding such copper core side wires to spark plug shells, which are commonly made from metal such as steel, has been problematic. For instance, U.S. Pat. No. 4,970,426 discloses welding a copper core side wire to the end of the shell with the copper enclosed within the electrode sheath. This technique is used since copper is an electrically highly conductive material which makes it difficult to resistance weld copper directly to steel without causing voids and embrittlement in the copper adjacent to the weld interface.
If the copper core side wire has internal voids and embrittlement, it may easily break off of the metal shell. These problems may also hinder heat transfer and hence fail to keep the firing tip cool to retard erosion (wear) and reduce thermal stress. Although the process of the '426 patent successfully joins the side wire to the spark plug shell, it does not yield optimum thermal conductivity since the copper core does not directly contact the steel shell.
As a further attempt to eliminate the void and embrittlement problem, U.S. Pat. No. 5,210,457 discloses inserting a second core of nickel alloy or iron within the copper core of the side wire and welding that end to the spark plug shell. While somewhat successful, this method also has drawbacks relating to thermal conductivity and complexity.
Still yet another method of welding copper core side wires to spark plug shells is disclosed in U.S. Pat. No. 5,530,313. As illustrated in FIG. 7, the '313 patent discloses a "stake" method of welding wherein the side wire sheath 300 penetrates deeply into the steel shell 310 to provide an anchor for enhancing mechanical strength. The weld profile for such a process shows that the penetrating stakes 320 of the sheath 300 penetrate the shell 310 essentially vertically and do not roll under the copper core 330. While the stake method is effective for joining the side wire to the shell, there is still room for improvement in the art.
In view of the foregoing, it would be desirable to provide a spark plug having a side wire with a copper core surrounded by a metal sheath which partially rolls over the end of the copper core at the weld interface for providing enhanced surface area for bonding and therefore high mechanical strength as well as direct contact between the copper core and the metal shell for enhancing thermal conductivity. It would also be desirable to provide a method of preparing a side wire stock for such a weld and a method for performing the weld.